1. Field of the Invention
The present invention generally relates to focusing acoustic waves and, more particularly, to focusing utilizing acoustic lenses.
2. Description of the Related Art
Concepts of a “superlens” and its development have expanded the understanding of wave propagation and imaging across all fields of science and engineering. It is well known, from the theory of diffraction, that focusing of waves is limited by diffraction and the spatial resolution is determined by the Rayleigh criterion. The concept of the superlens has provided an avenue to obtain resolution beyond the classical diffraction limit. In general, in electromagnetic waves, the superlensing effect originates from a negative μ and negative ε, leading to negative refractive index. While natural materials may not exhibit negative refractive indices, artificial structures with sub-wavelength spacing have demonstrated negative refractive indices. These new classes of structure that have a negative refractive index are known as metamaterials. An important consequence of the negative refractive index in wave propagation is that the “wave velocity” and the group velocity are in opposite direction to each other. The experimental demonstration of superlens effect in electromagnetic wave propagation has quickly spread into many fields including acoustics. Three dimensional acoustic metamaterials and structures have been developed, based on anisotropic wave propagation and observation of wave velocity and group velocity to be in opposite directions along specific directions in the structures.
Following the development of metamaterials, the possibility of achieving acoustic wave focusing beyond diffraction limit with two-dimensional structures has been explored. Many of these approaches use Helmholtz resonators or a split ring type resonator. Additionally, two-dimensional periodic and aperiodic arrays have demonstrated acoustic wave focusing. Some designs include two dimensional grating with aperiodic spacing, which is less than the wavelength of microwave radiation and have demonstrated focusing beyond the diffraction limit. Following similar arguments several groups have designed two dimensional grating structures to focus electromagnetic waves and have demonstrated focusing beyond the diffraction limit. Most of the acoustic gratings reported are parallel lines and the focusing has been along a line.
Typically focusing of acoustic waves has been achieved with lens structures based on refraction. Single element acoustic lenses based on refraction are an integral part of scanning acoustic microscopy (SAM). In SAM high frequency acoustic waves have been brought to focus with an acoustic lens on to the surface of a sample in presence of a coupling liquid. Single element acoustic lenses have been used to focus acoustic waves in presence of high pressure gases and in ambient air. The focal spot size in single element lenses has been limited by diffraction and the spatial resolution, s, as determined by the Rayleigh criterion, s=1.22(λ/D), where λ is the wavelength of sound in the coupling medium and D is the diameter of the lens, similar to the electromagnetic waves above. A single element acoustic lens typically consists of a cylindrical rod with a piezoelectric transducer attached at one end. The opposite end generally has a spherical curvature that contacts the coupling liquid. Although these structures have been demonstrated to have better resolution, they operated only in a narrow frequency range and had serious practical limitations.
Planar acoustic lens structures based on Fresnel diffraction have also been developed for focusing of acoustic waves. The focal spot size and spatial resolution of Fresnel lenses are also determined by diffraction theory and the Rayleigh criterion. Although Fresnel lenses have planar structure, the individual corrugations may have thickness variations or steps for matching the phase of the acoustic field at the focal spot. Acoustic Fresnel lenses may also consist of a cylindrical rod with corrugations on one end and a piezoelectric transducer at the opposite end. The corrugated structure is immersed in coupling fluid to focus acoustic waves on the sample surface. Both single element lenses and Fresnel lenses have been used to focus acoustic fields only in the far field.
Aperiodic grating structures have additionally been explored for acoustic focusing. Aperiodic grating structures are generally optimized to achieve acoustic focusing in the near field through a combination of near field diffraction and multiple scattering theories. Initially the concept was demonstrated by focusing an acoustic beam to a line by optimally arranged cylindrical rods. This has been extended to focus acoustic waves to circular spot by optimally arranging a ring structure with axial symmetry. This particular lens consisted of several rings of varying diameters arranged with the centers aligned along the line. The distance between rings, diameter of the rings and positioning of the rings was optimized in three dimensions using a genetic algorithm to operate at 2.2 kHz. Although the lens has a circular acoustic focal spot for acoustic imaging applications the three-dimensional structure was quite complicated.
Contemporary three dimensional and two dimensional periodic and aperiodic array structures have been designed to achieve subwavelength focusing. However, subwavelength resolution in a narrow band frequency range is commonly observed. Demonstration over broad frequency ranges has been limited. A cylindrical acoustic lens structure has been used to show continuous focusing over the 4.2-7 kHz frequency band with up to λ/4.1 focusing. However, this lens had focal regions at every demonstrated frequency. Other types of broadband tunable resonators were demonstrated based on anisotropic metafluids with structures consisting of corrugated, periodic cylinders in a fluid. These structures operated in the frequency range of 1-5 kHz, with up to 4 clearly defined resonances at which the amplitude of the acoustic pressure was high. An approach using a two dimensional periodic unit cell acoustic lens with a broad bandwidth and a graded refractive index medium was developed and demonstrated to operate in the range of 1.5-4.5 kHz. Multiband and broadband acoustic structures based on split hollow spheres have been demonstrated between 0.9-1.6 kHz. The multiband structure had three distinct resonances while the broadband structure had six distinct resonances. With the exception of the cylindrical acoustic lens, all the other acoustic lenses had bandwidths in the range of 0.9-5 kHz with very few resonances for evaluation of the spatial resolution. Subwavelength spatial resolution at each of the reported resonant frequencies was not clearly established.
Developing structures to focus acoustic waves to a circular spot in air could be important in acoustic imagining of materials. It is also expected that focusing such waves could significantly enhance capabilities of non-contact air coupled acoustic non-destructive evaluation. Accordingly, there is a need in the art for a broadband acoustic lens with clearly established subwavelength spatial resolution.